1. Field of the Invention
The present invention relates to an apparatus for irradiating a magnetooptical recording medium having Kerr effect or Faraday effect with linearly polarized light to reproduce the information recorded on said medium from the light transmitted or reflected by said medium.
2. Description of the Prior Art
In the field of apparatus for optically reproducing information recorded on a magnetooptical recording medium, there is already known, as disclosed in the Japanese Laid-open Patent Application no.No. 169947/1982, an apparatus of so-called differential method in which a laser beam light spot reflected or transmitted by a record face is divided into two optical paths according to the angle of plane of polarization, and two signals obtained by photoelectric conversion in thus divided two optical paths are supplied to a differential amplifier to mutually cancel the noise in phase, thereby obtaining the information signal alone.
More specifically, the light reflected or transmitted by the magnetooptical recording medium shows a change in the rotational angle of the plane of polarization by .+-..theta.k, respectively by Kerr effect or Faraday effect, according to the record direction of vertical magnetization in said recording medium. In the above-mentioned apparatus, said reflected or transmitted light enters a polarized beam splitter at a suitable angle for division into two light beams of varying intensity according to the rotational angle of polarization, and said two light beams are respectively converted, for example with photodiodes, into electrical signals for supply to the differential amplifier. In two input signals to the differential amplifier, the information signals are mutually inverted in the polarity of amplitude according to the rotational angle of plane of polarization, so that an output signal can be obtained by adding and amplifying two input signals. On the other hand, the noise components contained in the reproduced signals are solely dependent on the light intensity and are in phase, so that they can be mutually cancelled by differential amplification.
However, in order to securely eliminate the noise in such apparatus, the noise components in phase contained in two input signals to the differential amplifier should have the same electrical level. In the Japanese Laid-open Patent Application No. 169947/1982, the differential amplifier is provided, at an input terminal thereof of a higher signal level, with an attenuator to control the signal gain, thus bringing the noise components to the same level.
The noise level control with such attenuator inevitably involves an attenuation of the information signal, so that the differential amplification of the input signals may be unable to provide a sufficiently high S/N ratio in the information signal.